A. Field of the Invention
This invention relates to the subject of calibration of pumps, and in particular to calibration of pumps in which the inlet pressure of the pump effects the volume of fluid moved per revolution of pump. The method involves recording the inlet pressure during use of the pump and using the recorded pressures to calibrate the pump prior to the next operation of the pump. The method is particularly suitable for use in calibrating a blood pump found in an extracorporeal circuit of a dialysis machine.
B. Description of Related Art
Hemodialysis machines are used for treating patients suffering from inadequate kidney function. Hemodialysis machines include, among other things, an extracorporeal blood circuit typically comprising an arterial line, a blood pump, a dialyzer and a venous line. Blood is removed from the patient via the arterial line and pumped by the blood pump to the dialyzer, where blood-borne toxins and excess fluids are removed from the patient""s blood. The blood is then returned to the patient via the venous line. The blood pump in most dialysis machines is a peristaltic pump, wherein the pump segment is squeezed by a pair of rollers that move over a given length of pump segment tubing installed in the blood pump.
The quality of the dialysis treatment is directly related to the control over the amount of the patient""s blood that flows through the dialyzer. Blood flow inaccuracy, often due to inaccurate blood pump calibration, is well known in dialysis clinics. These inaccuracies can be due to the fact that the inlet pressure to the blood pump (typically measured in the arterial line) has a significant effect upon the stroke volume of peristaltic pumps. In addition, where the pump segment and entire extracorporeal circuit are reused and subject to heat disinfection cycles between use, the heat can affect the pump segment itself, thereby changing the characteristics of the pump segment in the next use of the extracorporeal circuit. This phenomenon again changes the stroke volume of the blood pump and makes precise calibration more difficult.
The present invention provides a way of precisely calibrating the pump which compensates or accounts for the variations in inlet pressure to the blood pump and which can account for changes in the blood pump segment over time. Although the specific embodiment described herein is for calibrating a peristaltic blood pump, it should be appreciated that the calibration method can be applied to any type of pump where an inlet pressure versus flow rate relationship exists. Additionally, the calibration method is applicable outside the field of dialysis.
A method is provided for operating a pump that is subject to repeated uses. The invention provides for calibrating the pump at an expected inlet pressure that is derived from previous measurements of inlet pressure during a prior use of the pump. The result is a more accurate calibration of the pump. Further, the accurate calibration is performed independent of the accuracy of the device that is used to measure inlet pressure.
In a representative embodiment, the pump is placed in a fluid conduit which has a pre-pump or inlet pressure sensor. The method involves recording the inlet pressure in the fluid conduit during use of the pump. For example, in a blood pump embodiment, during dialysis treatments the inlet pressure to the blood pump is measured periodically and recorded in a computer readable memory that is part of the control system for the dialysis machine. At any time when the system slows or stops the blood pump (such as in the case of an alarm), the pressure recordings are paused.
The method continues by deriving (e.g., calculating) from the recordings of inlet pressure a mean or average inlet pressure at the pump that occurred during the use of the pump. In the dialysis embodiment, the average pressure over the course of the dialysis treatment is calculated. Furthermore, over multiple uses of the pump, such as multiple dialysis sessions, the average inlet pressure over multiple dialysis treatments can be averaged or regressed to arrive at a predicted inlet pressure for the next treatment or use of the pump, a pressure referred to herein as Pc or calibration pressure.
The method continues by calibrating the flow rate of the pump, with the calibrating performed while the inlet pressure to the fluid conduit is set to the mean inlet pressure or the calibration pressure Pc (either directly, or by varying the pressure in regular periodic intervals around the calibration pressure Pc). This xe2x80x9csmart pointxe2x80x9d calibration is thus made at an inlet pressure that is likely to be experienced in the next use of the pump, and thus tends to produce a better and more accurate calibration since variation in pump flow rate as a function of inlet pressure is accounted for.
Calibration of the pump can by made by moving a known volume of fluid from a source of fluid, such as a reserve tank or ultrafiltration tank in a dialysis embodiment, at a predetermined pump RPM and sending the fluid to drain. Again, this step of moving the known volume of fluid is done while the inlet pressure to the pump is maintained at the expected inlet pressure for the next use of the pump. The time to move the fluid is automatically recorded. The known volume of fluid divided by the time yields the flow rate. The flow rate divided by RPM gives volume per revolution.
Typically, this calibrated value of volume per revolution is used by the control system software during the next dialysis treatment to control the amount of dialysis of the patient that takes place. Thus, the method continues by subsequently using the pump after having been calibrated as described. In a typical example, the pump is operated at a flow rate that is calibrated in accordance with the expected inlet pressure.
In another aspect of the invention, a method of calibrating a peristaltic blood pump of a hemodialysis machine is provided. The hemodialysis machine has an extracorporeal circuit comprising an arterial line, a venous line, and a dialyzer, wherein the blood pump pumps blood from a patient through the arterial line and dialyzer and delivers the blood back to the patient via the venous line. The method comprises the steps of:
recording inlet pressures in the arterial line during dialysis sessions for the patient;
deriving from the recorded inlet pressures a calibration inlet pressure Pc to be used for calibration of the blood pump;
adjusting the inlet pressure of the arterial line to substantially match the calibration inlet pressure Pc; and
moving a volume of fluid with the blood pump while maintaining the inlet pressure of the arterial line at the calibration inlet pressure Pc, to thereby derive a volume per revolution for the pump at the pressure Pc.
In a preferred embodiment, the calibration pressure calculated as described herein is verified by comparing it to the previous calibration number for the blood tubing set to see if the change is outside of a specified range. If this verification challenges the validity of the calibration, the calibration is repeated up to a predetermined number of times. If, after repeated trials, the calibration is still outside of the specified range, then the calibration is deemed to have failed. In that event, the blood tubing set is changed (i.e., replaced) and/or the blood pump is serviced.
It should be appreciated that the patient""s instantaneous arterial pre-pump blood pressure may vary over the course of the treatment, and thus the flow rate of the blood pump will also vary. However, if the mean pressure is accurately predicted (as described herein) then the mean flow rate will also be set accurately and the correct dose of dialysis delivered to the patient.
In an alternative embodiment, in addition to using the xe2x80x9csmart pointxe2x80x9d calibration of the blood pump to set the blood pump RPM during treatment, the inlet pressure measurements during the treatment may be used to actively control the RPM of the blood pump in order to achieve a constant flow rate over varying pressure.
These and other aspects of the invention will be apparent from the following description of the presently preferred embodiment of the invention.